NEW YORK - Thousands of cases of breast and colon cancers might be averted each year if people in colder climates raised their vitamin D levels, researchers estimate in a new report.

A number of studies have suggested that vitamin D may be important in cancer risk. Much of this research is based on cancer rates at different latitudes of the globe; rates of breast, colon and ovarian cancer, for example, are lower in sunnier regions of the world than in Northern climates where cold winters limit people's sun exposure.

Sunlight triggers the synthesis of vitamin D in the skin, and people who get little sun exposure tend to have lower stores of the vitamin.

Complementing these studies are lab experiments showing that vitamin D helps prevent cancer cells from growing and spreading, as well as some clinical trials in which people given high doses of vitamin D showed lower cancer risks.

For the new study, researchers at the University of California used data on average wintertime blood levels of vitamin D and rates of breast and colon cancers in 15 countries.

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They found that rates of the diseases tended to fall as average vitamin D levels climbed, according to their report in the journal Nutrition Reviews. The protective effect against colon cancer seemed to begin when blood levels of vitamin D reached 22 nanograms per milliliter (ng/mL); for breast cancer, that number was 32 ng/mL.

The average late-winter vitamin D level among Americans is 15 to 18 ng/mL, according to the researchers.

They argue that, based on their data, if Americans were able to maintain a vitamin D level of at least 55 ng/mL, 60,000 cases of colon cancer and 85,000 cases of breast cancer could be prevented every year. Worldwide, those figures could be 250,000 and 350,000, respectively.

"This could be best achieved with a combination of diet, supplements and short intervals -- 10 or 15 minutes a day -- in the sun," lead study author Dr. Cedric F. Garland, a cancer prevention specialist at the University of California San Diego, said in a statement.

No one is recommending that people bake in the sun to reach high vitamin D blood levels. According to Garland, spending a matter of minutes in the midday sun, with 40 percent of the skin exposed, is enough. For fair-skinned people, the researchers estimate that just 3 minutes in the sun can be adequate, while darker-skinned people may need about 15 minutes.

A lifeguard in Southern California, Garland said, may have little need for extra vitamin D to reach potentially protective levels, whereas a Northerner who tends to stay indoors much of the year may need much more.

Garland and his colleagues recommend that, in addition to modest sun exposure, adults get 2,000 IU of vitamin D per day -- which is the "tolerable upper intake level" set by U.S. health officials.

That limit exists because of the risk of vitamin D toxicity, which causes elevated calcium levels in the blood and problems such as nausea, weight loss, fatigue and kidney dysfunction.